1. Field of the Invention
The present invention relates to a coating method, more particularly, to an improvement in extrusion type coating method that is used for manufacturing photosensitive materials and magnetic recording media.
2. Description of the Related Art
Photosensitive materials and magnetic recording media are manufactured through a coating step of forming a coating film by applying a coating liquid, such as a magnetic liquid, to a continuously running substrate sheet (hereinafter referred to as xe2x80x9cwebxe2x80x9d). In recent years, magnetic recording media such as magnetic recording tapes particularly for broadcasting and computer use have been rapidly improved in capacity and recording density, and there has been needed a coating technology capable of providing a magnetic layer which has an extremely thin film thickness and a smooth surface.
Coating apparatuses for applying a coating liquid to the surface of a web include, for example, apparatuses of types such as roll coater, gravure coating, roll coating plus doctor roll, extrusion coating, and slide coating. In recent years, an apparatus of extrusion type has been commonly used to apply a magnetic coating liquid.
One method employing an extrusion type coating apparatus in which the tip of a coating head is pushed against a web, as described in Japanese Patent Application Publication No. 58-109162, has been commonly used in the field of manufacturing magnetic recording media because a thin and uniform coating layer can be obtained by increasing the liquid pressure at the tip of the coating head utilizing the web tension, thereby eliminating the air accompanying the web. However, even in the method of applying a single coat of liquid using this type of coating head, the reduction in coating thickness is limited.
As a method for forming a much thinner coating layer, there is a method in which a very thin coating layer can be formed by applying an excess amount of coating liquid to a web using a web-pushing type extrusion coating apparatus and then scraping off the excess coating liquid with a blade disposed on the downstream side of the extrusion coating apparatus, as described in Japanese Patent Application Publication No. 7-287843. In this case, a device which applies an excess amount of coating liquid to a web is not necessarily limited to the web-pushing type extrusion coating apparatus, but coating apparatuses such as roller coater, gravure coater, and extrusion coater equipped with a back-tip roll can be used.
In the scraping-off type coating apparatus as disclosed in Japanese Patent Application Publication No. 7-287843, however, since the coating liquid recovered by being scraped off with the blade has once been exposed to the atmosphere in a process from the coating apparatus to the blade, the solvent contained in the coating liquid volatilizes, whereby the liquid physical properties such as viscosity and solid content concentration are changed. Thus, the apparatus has a disadvantage in that the recovered liquid must undergo treatment for adjusting its physical properties such as viscosity and solid content concentration before being reused. Further, the apparatus has a disadvantage in that dust is liable to be included in the coating liquid having been applied to the web because the recovered liquid comes once into contact with the atmosphere, so that the recovered liquid cannot be reused because dust cannot be removed even if an excess liquid recovered with the blade is filtered.
With such a background, the present inventors have proposed coating method and apparatus in Japanese Patent Application No. 2001-302402 corresponding to U.S. patent application Ser. No. 10/253,427 and German Patent Application No. 102 44 882.5 such that two slits of a coating slit and a recovering slit are provided so that an excess of coating liquid that has been discharged excessively from the coating slit and applied to a web is sucked up into the recovering slit. According to the coating method and apparatus, the recovered coating liquid can be reused without liquid-adjusting treatment and filtering treatment.
However, the coating method using two slits of the coating slit and recovering slit is a novel coating method exceeding the limit of conventional idea, and it is hence necessary to establish properly optimized coating conditions so as to improve the surface quality of the obtained coating layer. In particular, the pressure condition for sucking up an excess of coating liquid through the recovering slit is of importance in obtaining a coating layer with a very small thickness and high surface quality.
The present invention has been made in view of the above situation, and an object thereof is to provide a coating method in which in coating with two slits of a coating slit and a recovering slit, the pressure condition for sucking up an excess of coating liquid with a recovering slit is optimized so that a coating layer with a very small and even thickness can be obtained.
To attain the above object, the present invention is directed to a coating method in which in a state where a continuously running web is brought close to a lip surface at a tip of a coating head, a coating liquid sent into the coating head in excess of a desired amount is discharged through a coating slit and is applied to the web, and an excess of coating liquid applied to excess is sucked up through a recovering slit provided on a downstream side of the coating slit in a running direction of the web, wherein: a liquid pressure P (MPa) at a suction port of the recovering slit is adjusted in a range of 0.05 greater than Pxe2x89xa70.
According to the present invention, in the coating method using two slits of the coating slit and the recovering slit, the liquid pressure P (MPa) at a suction port of the recovering slit is adjusted in the range of 0.05 greater than Pxe2x89xa70, more preferably 0.02 greater than Pxe2x89xa70. Thus, coating thickness variations in the web width direction of the coating layer formed on the web can be restrained, and score-like coating omission can be prevented from occurring on the coating layer surface.
To attain the above object, the present invention is also directed to a coating method in which in a state where a continuously running web is brought close to a lip surface at a tip of a coating head, a coating liquid sent into the coating head in excess of a desired amount is discharged through a coating slit and is applied to the web, and an excess of coating liquid applied to excess is sucked up by a suction pump through a recovering slit provided on a downstream side of the coating slit in a running direction of the web, wherein: a pressure PIN (MPa) on a suction opening side of the suction pump is adjusted as PIN greater than xe2x88x920.02.
According to the present invention, in the coating method using two slits of the coating slit and the recovering slit, the pressure PIN (MPa) on the suction port side of the suction pump for sucking up an excess of coating liquid is adjusted as PINxe2x89xa7xe2x88x920.02, more preferably PINxe2x89xa7xe2x88x920.01. Thus, thickness variations in the web running direction of the coating layer formed on the web can be restrained. In this case, it is preferable that when a pressure loss xcex94P (MPa) from the suction port of the recovering slit to the suction opening of the suction pump is adjusted as xcex94P less than 0.07, more preferably xcex94P less than 0.03.
The coating slit and the recovering slit may be formed in the same coating head. Alternatively, the configuration may be such that the coating head is formed with the coating slit only, and the recovering head formed with the recovering slit is provided separately to integrally arrange the coating head and the recovering head. Also, as a web used in the present invention, a web in which an undercoat is applied and solidified in advance may be used.